Abstract

Vascular development in the mammalian retina is a paradigm for CNS vascular development in general, and its study is revealing fundamental mechanisms that explain the efficacy of antiangiogenic therapies in retinal vascular disease. During development of the mammalian retina, hypoxic astrocytes are hypothesized to secrete VEGF, which attracts growing endothelial cells as they migrate radially from the optic disc. However, published tests of this model using astrocyte-specific deletion of Vegf in the developing mouse retina appear to contradict this theory. Here, we report that selectively eliminating Vegf in neonatal retinal astrocytes with a Gfap-Cre line that recombines with approximately 100% efficiency had no effect on proliferation or radial migration of astrocytes, but completely blocked radial migration of endothelial cells, strongly supporting the hypoxic astrocyte model. Using additional Cre driver lines, we found evidence for essential and partially redundant actions of retina-derived (paracrine) and astrocyte-derived (autocrine) VEGF in controlling astrocyte proliferation and migration. We also extended previous studies by showing that HIF-1α in retinal neurons and HIF-2α in Müller glia play distinct roles in retinal vascular development and disease, adding to a growing body of data that point to the specialization of these 2 hypoxia-sensing transcription factors.

Figure 9

(A) Dorsal views of P14 brains. In the phenotypically WT Gfap-CreVegffl/+ brain (left), the cerebral cortices cover most of the midbrain. In the Gfap-CreVegffl/fl brain (right), the cortex is hypoplastic and the midbrain is visible. Scale bar: 5 mm. (B) Localization of vasculature (GSL staining; left panel) and Cre activity (right 3 panels) in coronal sections of E17 brains at the level of the anterior commissure. Cre activity, visualized with the R26-LSL-mtdT-2A-H2B-GFP reporter, shows mtdT and H2B-GFP in ventricular cells and radial glia. The Gfap-CreVegffl/fl dorsal cortex is thin and hypovascular. In the left 3 panels, the midline is near the left border of each image. Right panel shows enlargements of the regions delineated by the squares in the central 2 columns, with merged red and green channels. Scale bars: 500 μm (left 3 panels) and 250 μm (far-right panel).